Organic EL device and method of manufacturing organic EL device

ABSTRACT

In a method of manufacturing an organic EL device by using an ink jet method, a discharge amount of an ink composition for a light emitting layer is made to be greater than a discharge amount of an ink composition for a hole injection/transportation layer, so that a film formation region of the light emitting layer becomes equal to, or greater than, a film formation region of the hole injection/transportation layer.

This is a Continuation Division of U.S. patent application Ser. No.09/820,728 filed Mar. 30, 2001 now U.S. Pat. No. 6,852,994. The entiredisclosure of the prior application is hereby incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an organic EL (electro-luminescence) devicethat operates as an electric light emitting device for use in a display,a display light source, etc, and to a method of manufacturing thereof.

2. Description of Related Art

Recently, development has been accelerated of organic EL display devicesusing organic materials as spontaneous light emitting displays that willreplace liquid crystal displays. Organic EL devices using the organicmaterials are principally described in the reference “Appl. Phys. Lett.”51(12), 21 Sep., 1987, p 913, that discloses a vacuum deposition methodof organic materials having a low molecular weight, and in the reference“Appl. Phys. Lett.” 71(1), 7 Jul., 1997, p. 34 et seq., that disclosesan application method of organic materials having a high molecularweight.

In order to achieve color display in the case of the low molecularweight materials, a method that vacuum deposits different light emittingmaterials onto desired pixels through a mask has been employed. As tothe high molecular weight materials, on the other hand, a coloringmethod using an ink jet method has drawn attention because it canexecute patterning delicately and easily. Examples of the formation oforganic EL devices by the ink jet method are disclosed in JapanesePatent Laid-Open Nos. 235378/1995, 12377/1998, 153967/1998, 40358/1999,54270/1999 and 339957/1999.

A device structure includes a hole injection/transportation layer formedin many cases between an anode and a light emitting layer to enhancelight emission efficiency and durability (“Appl. Phys. Lett.” 51, 21Sep., 1987, p 913). The term “hole injection/transportation layer” usedherein is a generic term of a layer that has a function of injecting andtransporting holes from the anode side to a light emitting layer.According to the related art technology, a film of a buffer layer or ahole injection/transportation layer is formed by using conductivepolymers, such as polythiophene derivatives or polyaniline derivatives,by an application method, such as spin coating (“Nature”, 357, 477,1992). The hole injection/transportation layer can be formed by vacuumdepositing phenylamine derivatives as the low molecular weight material.

An ink jet system is extremely effective to easily form a film having aminiature pattern without wasting organic thin film materials.

However, when an organic EL device, having a laminate structure, isformed in accordance with the ink jet system and moreover, when thelaminate structure includes a hole/injection layer plus a light emittinglayer, for example, an underlying layer, as the conductive layer, isexposed when a cathode is formed, if an application region of the holeinjection/transportation layer, as the underlying layer, is broader thanthe application region of the light emitting layer as an upper layer,with the result that a current leaks and the resulting device has lowefficiency.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention, when manufacturingan organic EL device having a laminate structure by an ink jet method,to provide a high efficiency organic EL device that is free from acurrent leak. It is also an object of the invention to provide a methodof manufacturing such a high efficiency organic EL device.

The present invention provides an organic EL device having a structurein which a laminated film, of at least two layers, is formed by an inkjet system, and which includes a hole injection/transportation layer anda light emitting layer, a film formation region of the light emittinglayer being equal to, or greater than, a film formation region of thehole injection/transportation layer.

The present invention also provides a method of manufacturing an organicEL device having a structure in which a laminated film, of at least twolayers, is formed by an ink jet system, and which includes a holeinjection/transportation layer and a light emitting layer, such that,when a discharge amount of an ink composition that forms the holeinjection/transportation layer is A, and a discharge amount of an inkcomposition that forms the light emitting layer is B, a relationship ofA≦B is satisfied.

The present invention also provides a method of manufacturing an organicEL device having a structure in which a laminated film, of at least twolayers, is formed by an ink jet system, and which includes a holeinjection/transportation layer and a light emitting layer, such that,when a sum of discharge amounts of an ink composition that forms thehole injection/transportation layer is A, and a sum of discharge amountsof an ink composition that forms the light emitting layer is B, arelationship of A≦B is satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a step of a method of manufacturingan organic EL device according to one embodiment of the presentinvention;

FIG. 2 is a sectional view showing another step of the method ofmanufacturing the organic EL device according to one embodiment of thepresent invention;

FIG. 3 is a sectional view showing another step of the method ofmanufacturing the organic EL device according to one embodiment of thepresent invention;

FIG. 4 is a sectional view showing still another step of the method ofmanufacturing the organic EL device according to one embodiment of thepresent invention;

FIG. 5 is a sectional view showing still another step of the method ofmanufacturing the organic EL device according to one embodiment of thepresent invention;

FIG. 6 is a sectional view showing still another step of the method ofmanufacturing the organic EL device according to one embodiment of thepresent invention;

FIG. 7 is a sectional view showing a structural example of an organic ELdevice;

FIG. 8 is a sectional view showing a substrate structure used in oneembodiment according to the present invention;

FIG. 9 is a graph for comparing a voltage-light emission efficiencycharacteristic of an organic EL device according to an embodiment of thepresent invention;

FIG. 10 is a graph for comparing a voltage-current characteristic of anorganic EL device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an organic EL device having a structure in which at least twolaminated films are formed by an ink jet system, and which contains ahole injection/transportation layer and a light emitting layer, thepresent invention has a feature in that a film formation region of thelight emitting layer is equal to, or greater than, a film formationregion of the hole injection/transportation layer.

In the present specification, the term “film formation region of lightemitting layer is equal to, or greater than, film formation region ofhole injection/transportation layer” means, for example, that the filmformation area or volume of the light emitting layer, as viewedplane-wise, is equal to, or greater than, the film formation area orvolume of the hole injection/transportation layer, as viewed plane-wise.

The structure described above can prevent leakage between the holeinjection/transportation layer and the cathode, and can provide anorganic EL device having high efficiency.

In a method of manufacturing an organic EL device having a structure inwhich at least two layers of laminated films are formed by an ink jetsystem, and which includes a hole injection/transportation layer and alight emitting layer, the present invention has another feature in that,when a discharge amount of an ink composition that forms the holeinjection/transportation layer is A, and a discharge amount of an inkcomposition that forms the light emitting layer is B, a relationship ofA≦B is satisfied.

According to such a method of manufacturing an organic EL device, therelationship of A≦B is satisfied between the discharge amount A of theink composition that forms the hole injection/transportation layer, andthe discharge amount B of the ink composition when the light emittinglayer is formed. When this condition is satisfied, the film formationregion of the light emitting layer can be made equal to, or greaterthan, the film formation region of the hole injection/transportationlayer. Accordingly, an organic EL device having high efficiency andbeing free from leakage can be provided when manufacturing an organic ELdevice having a laminate structure and produced by an ink jet system.

In a method of manufacturing an organic EL device having a structure inwhich at least two layers of laminated films are formed by an ink jetsystem, and which contains a hole injection/transportation layer and alight emitting layer, the present invention has another feature in that,when the sum of discharge amounts of an ink composition that forms thehole injection/transportation layer is A, and the sum of dischargeamounts of an ink composition that forms the light emitting layer is B,the relationship of A≦B is satisfied.

According to such a method of manufacturing an organic EL device, therelationship of A≦B is satisfied when the sum of the discharge amountsof the ink composition that forms the hole injection/transportationlayer is A, and the sum of the discharge amounts of the ink compositionthat forms the light emitting layer is B. When this condition issatisfied, it is possible to enhance patterning accuracy, to set thefilm formation region of the light emitting layer to be equal to, orgreater than, the film formation region of the holeinjection/transportation layer, and to provide an organic EL devicehaving high efficiency and being free from the leakage. Since patterningaccuracy is high, light emission with higher uniformity can be acquiredin a panel, or the like, having a large number of pixels.

Hereinafter, an embodiment of the present invention will be explained indetail with reference to the drawings.

The method of manufacturing an organic EL device by the ink jet systemincludes the steps of preparing respective ink compositions bydissolving or dispersing materials of hole/injection transportationlayer, made of organic materials that form the device, and lightemitting materials in a solvent, discharging the ink composition from anink jet head to form a pattern on a transparent substrate, and formingthe hole injection/transportation layer and the light emitting materiallayer.

FIG. 1 is a sectional view of a substrate used for manufacturing anorganic EL device by the ink jet system. ITO 11 is patterned astransparent pixel electrodes on a glass substrate 10 or a substrateequipped with TFT. Partitions (hereinafter called “banks”) 13, thatinclude SiO₂ 12 and an organic ink-repellent material, or an organicmaterial rendered ink-repellent, are disposed in regions that partitionthe pixels. The shape of these banks, that is, the open shape of thepixels, may be any of a circle, an ellipse, a rectangle or a stripe.Since the ink composition has a surface tension, the corners of therectangle are preferably rounded.

FIGS. 2 to 6 show a laminate layer of a hole injection/transportationlayer+light emitting layer, and a method of manufacturing a device bythe ink jet system. An ink composition 14, containing a holeinjection/transportation material, is discharged from an ink jet head 15and is applied into a pattern. After the ink composition is applied, thehole injection/transportation layer 16, is formed by removing thesolvent, and/or conducting heat treatment, or causing a nitrogen gas toflow.

Subsequently, an ink composition 17, containing the light emittingmaterials, is applied onto the hole injection/transportation layer, andthe light emitting layer 18 is formed by removing the solvent, and/orconducting heat treatment, or causing a nitrogen gas to flow.

Thereafter, a cathode 19 is formed by vacuum deposition or sputtering ofa metal, such as Ca, Mg, Ag, Al, Li. To protect the device, a seal layer20 is further formed from an epoxy resin, an acrylic resin or liquidglass, and the device is completed. An electrode, made of a reflectivemetal material, is used as an anode in place of the transparentelectrode 11. A layer of a co-evaporation material, such as Ag or Mg, isdisposed as the cathode. Thus, an organic EL device, of the type fromwhich light travels through the cathode side, can be acquired.

FIGS. 6 and 7 show the sectional structure of the resulting device. Inthe device structure shown in FIG. 7, the hole injection/transportationlayer 21 and the cathode 23 are kept in contact with each other, andcurrent leakage develops and lowers device characteristics. To preventthe current leakage, it is necessary, for the structure of the organicEL device, that the film formation region of the light emitting layer 18be equal to, or greater than, the film formation region of the holeinjection/transportation layer 16, as shown in FIG. 6. When an electroninjection/transportation layer is further to be laminated, the currentleakage does not occur, even when the film formation region of the lightemitting layer 18 is smaller than the film formation region of the holeinjection/transportation layer 16, provided that the film formationregion of the electron injection/transportation layer is equal to, orgreater than, the film formation region of the holeinjection/transportation layer. However, a drawback occurs in that thelight emitting region becomes narrower.

To form a laminated film that is free from the current leakage, it isnecessary to satisfy the relationship of A≦B with A representing thedischarge amount of the ink composition for the holeinjection/transportation layer, and B representing the discharge amountof the ink composition for the light emitting layer when the applicationof the ink composition is made once per pixel (by one droplet). When theamounts of A and B are reduced and the ink compositions areconsecutively applied a large number of n and m times per pixel,respectively, the sums of the discharge amounts must satisfy therelationship of nA≦mB. These A and B amounts may well be adjustedsuitably in accordance with the size of the pixels and the specificationof the ink jet head (nozzle diameter, etc) used.

The present invention can be applied to organic EL devices of both anactive matrix system and a passive matrix system.

The present invention will be explained in further detail with referenceto the following examples, but is not particularly limited thereto.

EXAMPLE 1

FIG. 8 shows a substrate used in this example. Though the drawing showsonly one pixel, these pixels were arranged in a 70.5 μm pitch. A bankwas formed from a laminate layer of polyimide 27 and SiO₂ 28 byphotolithography on a glass substrate 26 on which ITO 25 was patterned.A bank diameter (open diameter of SiO₂) was 28 μm and a height was 2 μm.Opening at the uppermost part of the polyimide bank was 32 μm. Before anink composition of a hole injection/transportation material was applied,the polyimide bank 27 was subjected to ink-repelling treatment byatmospheric plasma treatment. The condition of the atmospheric plasmatreatment was at an atmospheric pressure and power of 300 W, with adistance between an electrode and the substrate being 1 mm. In oxygenplasma treatment, an oxygen gas flow rate was 80 ccm, a helium gas flowrate was 10 SLM and a table conveying speed was 10 mm/sec. In subsequentplasma CF₄ processing, a CF₄ gas flow rate was 100 ccm, a helium gasflow rate was 10 SLM and a table conveying speed was 5 mm/sec. Acomposition shown in Table 1 was prepared as an ink composition for ahole injection/transportation layer.

TABLE 1 Ink composition for hole injection layer content compositionmaterial name (wt %) hole injection/ PEDT/PSS (“,aytron P”) (aqueous7.25 transportation material dispersion) polar solvent water 52.75methanol 5 isopropyl alcohol 5 1,3-dimethyl-2-imidazolidinone 30 silanecoupling agent γ-glycidyloxypropyltrimethoxysilane 0.08

After the surface treatment of the substrate, 15 pl of the inkcomposition for the hole injection/transportation layer, tabulated inTable 1, was discharged and applied into a pattern from a head (modelMJ-930C of Epson Co.) of an ink jet printing apparatus. The solvent wasremoved at 1 Torr and a room temperature for 20 minutes, and heattreatment was then conducted in the open air at 200° C. (on a hot plate)for 10 minutes to form the hole injection/transportation layer.

A composition tabulated in Table 2 was prepared as an ink compositionfor the light emitting layer.

TABLE 2 Ink composition for light emitting layer (green) contentcomposition material name (wt %) light emitting material PPV precursorsolution (1.5 wt %) 20 (water/methanol = 5/95 mixture solution) polarsolvent 1,3-dimethyl-2-imidazolidinone 70 butylcarbitol acetate 10

First, 20 pl of the ink composition for the light emitting layer,tabulated in Table 2, was discharged and applied into a pattern from ahead (model MJ-930C of Epson Co.) of an ink jet printing apparatus.After the solvent was removed at 1 Torr and a room temperature for 20minutes, heat treatment was conducted at 150° C. for 4 hours in anitrogen atmosphere to conjugate the composition and to form a greenlight emitting layer.

Ca and Al were sputtered to thickness of 20 nm and 200 nm, respectively,to form a cathode, and sealing was finally conducted by using an epoxyresin. The device fabricated by this example was referred to as “device(1)”.

A color device and a color panel can be fabricated by dividedly formingdifferent pixels by using ink compositions containing light emittingmaterials having different light emission colors, such as green, red andblue.

EXAMPLE 2

A device was fabricated in the same way as in Example 1, with the onlyexception that the discharge amounts of the ink composition for the holeinjection/transportation layer and the ink composition for the lightemitting layer were changed. The discharge amounts were 15 pl for theink composition for the hole injection/transportation layer, and 15 plfor the ink composition for the light emitting layer, too. The devicefabricated by this Example was referred to as “device (2)”.

EXAMPLE 3

A device was fabricated in the same way as in Example 1, with the onlyexception that the discharge amounts of the ink composition for the holeinjection/transportation layer and the ink composition for the lightemitting layer and the number of times of their discharging werechanged.

The ink composition for the hole injection/transportation layer in anamount of 5 pl was continuously discharged three times to the samepixel, while the ink composition for the light emitting layer (Table 1)in an amount of 10 pl was continuously discharged two times to the samepixel. When each composition was discharged several times by reducingits discharge amount, the diameter of the droplet became smaller.Because the droplets that had already been discharged and stored insidethe pixel pulled subsequent droplets, accuracy of the impact point canbe enhanced. The device fabricated in this embodiment was referred to as“device (3)”.

EXAMPLE 4

A composition tabulated in Table 3 was prepared as the ink compositionfor the light emitting layer.

TABLE 3 Ink composition for light emitting layer (green) contentcomposition material name (wt %) light emitting material PPV precursorsolution (1.5 wt %) 30 (water/methanol = 5/95 mixture solution) polarsolvent 1,3-dimethyl-2-imidazolidinone 60 butylcarbitol acetate 10

The ink composition for the hole injection/transportation layer was thesame as that used in Examples described above (Table 1). Next, 20 pl ofthe ink composition for the hole injection/transportation layer and 10pl of the ink composition for the light emitting layer were discharged.The other conditions were the same as those of Example 1. The devicefabricated in this example was referred to as “device (4)”.

FIGS. 9 and 10 show current-voltage characteristics andefficiency-voltage characteristics of the devices (1) and (4) fabricatedin Examples 1 and 4, respectively. In the device (4), a current leakagewas observed in a low voltage region below a threshold voltage (Vth).For this reason, a curve of light emission efficiency rose more smoothlythan in other devices not having the leak, and efficiency was lower,too. In the devices (1) to (3), high efficiency devices free from thecurrent leak could be obtained.

When a substrate having 200×200 pixels was formed under the sameconditions as those of Example 3, a green light emitting device that wasuniform throughout the entire surface could be obtained.

EXAMPLE 5

Next, an example using materials that were soluble in an organic solventwas represented as the ink composition for the light emitting layer.Though this example used polydioctyl fluorine as a blue color lightemitting layer, the present invention is not limited to polydialkylfluorenes, and instead can also use polydialkylfluorene derivatives andpolyparaphenylene vinylene derivatives, for example.

A composition as an ink composition for a light emitting layer tabulatedin Table 4 and a composition as an ink composition for a holeinjection/transportation layer tabulated in Table 5 were respectivelyprepared.

TABLE 4 Ink composition for hole injection layer content compositionmaterial name (wt %) hole injection/ PEDT/PSS (“Pytron P”) 7.25transportation material (aqueous dispersion) PSS (polystyrenesulfonicacid) 0.94 polar solvent water 51.81 methanol 5 isopropyl alcohol 51,3-dimethyl-2-imidazolidinone 30

TABLE 5 Ink composition for light emitting layer (blue) compositionmaterial name content light emitting material polydioctyl fluorine 1 gnon-polar solvent cyclohexylbenzene 100 ml

After the plasma treatment was conducted in the same way as in Example1, 15 pl of the ink composition for the hole injection/transportationlayer (Table 4) was discharged and applied into a pattern. The solventwas removed in vacuum (1 Torr) and at room temperature for 20 minutes.Heat treatment was then conducted in open air at 200° C. (on a hotplate) for 10 minutes, thereby forming the hole injection/transportationlayer.

Next, 20 pl of the ink composition for the light emitting layer (Table5) was discharged and applied into a pattern, and the solvent wasremoved in vacuum (1 Torr) and at room temperature for 20 minutes. Heattreatment was then conducted in a N₂ atmosphere at 50° C. for 20minutes, thereby forming the light emitting layer. As a cathode, Ca wasvacuum evaporated to a thickness of 20 nm and Al was sputtered to 200nm. Finally, sealing was conducted by using an epoxy resin. This device,too, provided a high efficiency device free from the current leakage inthe same way as the device of Example 1. In the device fabricated byusing the same ink compositions and discharging 20 pl of the inkcomposition for the hole injection/transportation layer and 10 pl of theink composition for the ink composition, the current leakage wasobserved even below a threshold voltage, and its light emissionefficiency was lower than that of the device described above.

As described above, when fabricating an organic EL device having alaminate structure by an ink jet system, the present invention sets adischarge amount of an ink composition for a light emitting layer to begreater than a discharge amount of an ink composition for a holeinjection/transportation layer. In this way, the present invention canmake a film formation region of the light emitting layer to be greaterthan the film formation region of the hole injection/transportationlayer, and can provide an excellent organic EL device that is free froma current leakage and having high light emission efficiency.

1. A method of manufacturing an organic EL device, comprising the stepsof: providing a first composition from an ink jet head, the firstcomposition forming a hole injection/transportation layer and includinga solvent; forming the hole injection/transportation layer by removingthe solvent from the first composition; providing a second compositionfrom an ink jet head, the second composition forming a light emittinglayer and including a solvent; forming the light emitting layer byremoving the solvent from the second composition, a relationship of A≦Bbeing satisfied, A being a amount of the first composition, and B beinga amount of the second composition.
 2. An organic EL device manufacturedaccording to the method of claim
 1. 3. A method of manufacturing anorganic EL device, comprising the steps of: providing a firstcomposition from an ink jet head, the first composition forming a holeinjection/transportation layer and including a solvent; forming the holeinjection/transportation layer by removing the solvent from the firstcomposition; providing a second composition from an ink jet head, thesecond composition forming a light emitting layer and including asolvent; forming the light emitting layer by removing the solvent fromthe second composition, a relationship of A≦B being satisfied, A being asum of discharge amounts of the first composition, and B being a sum ofdischarge amounts of the second composition.
 4. An organic EL devicemanufactured according to the method of claim
 3. 5. A method ofmanufacturing an organic EL device, comprising the steps of: forming apartition above a substrate; providing a first composition in a regionbounded by the partition from an ink jet head, the first compositionforming a hole injection/transportation layer and including a solvent;forming the hole injection/transportation layer by removing the solventfrom the first composition; providing a second composition in the regionbounded by the partition from an ink jet head, the second compositionforming a hole light emitting layer and including a solvent; forming thelight emitting layer by removing the solvent of the second composition,a relationship of A≦B being satisfied, A being an amount of the firstcomposition provided in the region bounded by the partition, and B beingan amount of the second composition provided in the region bounded bythe partition.